The spore-forming bacterium Bacillus anthracis causes anthrax, and is classified as one of six Category A agents considered as major threats as a bioweapon. Because of its pivotal role in disease pathogenesis, a number of strategies to inhibit anthrax toxin are currently under development, including monoclonal antibody- based therapies. However, there is significant concern about the ease with which the bacterium may be engineered to avoid vaccine protection or antitoxin therapy, e.g. by removing antibody-binding sites from the protective antigen (PA) toxin subunit. A next-generation strategy for antitoxin development, one that addresses this limitation, involves the use of a soluble receptor decoy inhibitor (RDI);Presumably PA cannot be engineered to evade cellular receptor recognition and therefore the RDI should be effective even against forms of PA that have been deliberately altered to resist antibody neutralization. We have recently developed an RDI which has many properties desirable in a broadly acting anthrax therapeutic: it binds to the receptor-binding site of PA with an affinity that is on a par with some of the leading therapeutic antibodies (Kd = 0.2nM);it blocks intoxication via both known cellular receptors for anthrax toxin;its dissociation rate from PA is extremely slow (t1/2 complex = 15 hours);it is non-immunogenic;its production is easily scaleable using a bacterial expression system;it can neutralize PA at stoichiometric concentrations and protects rats against toxin killing. This research plan represents a comprehensive strategy for advancing the RDI as a candidate therapy for anthrax. We will characterize and optimize its pharmacokinetic properties by disrupting its interaction with its physiological ligands (collagen IV and laminin), and by exploiting PEGylation and Ig fusion protein approaches. We will also establish if this class of inhibitor is effective at neutralizing antibody-resistant forms of PA, as expected. Moreover, we will establish if this class of inhibitor can prevent disease in mice caused by Sterne spores that express either wild-type or antibody-resistant PA. These experiments will set the stage for future studies aimed at establishing the effectiveness of the RDI in preventing disease caused by highly virulent strains of B. anthracis. We anticipate that the RDI will be a useful adjunct anthrax therapy that could potentially synergize with monoclonal antibodies to treat infections caused by wild-type bacterial strains while at the same time providing a straightforward strategy for dealing with engineered, weaponized bacterial strains.